Aqueous solution, Claisen rearrangements

Nearly simultaneous with reports of the aqueous-solution Claisen rearrangement from the Grieco group was a publication from Broka revealing the Claisen rearrangement of the following vinyl ether [25] ... 

To obtain a maximum yield of the acid it is necessary to hydrolyse the by-product, iaoamyl iaovalerate this is most economically effected with methyl alcoholic sodium hydroxide. Place a mixture of 20 g. of sodium hydroxide pellets, 25 ml. of water and 225 ml. of methyl alcohol in a 500 ml. round-bottomed flask fitted with a reflux (double surface) condenser, warm until the sodium hydroxide dissolves, add the ester layer and reflux the mixture for a period of 15 minutes. Rearrange the flask for distillation (Fig. II, 13, 3) and distil off the methyl alcohol until the residue becomes pasty. Then add about 200 ml. of water and continue the distfllation until the temperature reaches 98-100°. Pour the residue in the flask, consisting of an aqueous solution of sodium iaovalerate, into a 600 ml. beaker and add sufficient water to dissolve any solid which separates. Add slowly, with stirring, a solution of 15 ml. of concentrated sulphuric acid in 50 ml. of water, and extract the hberated acid with 25 ml. of carbon tetrachloride. Combine this extract with extract (A), dry with a httle anhydrous magnesium or calcium sulphate, and distil off the carbon tetrachloride (Fig. II, 13, 4 150 ml. distiUing or Claisen flask), and then distil the residue. Collect the wovaleric acid 172-176°. The yield is 56 g. 

The Claisen rearrangement of allyl vinyl ether in aqueous solution was studied by Gao et al. [157] by using the gas-phase ab initio MEP determined previously by the Jorgensen s group [60], In this case, 69 of the 143 frames were employed along the perturbation procedure in the isothermal-isobaric ensemble at 25 °C and 1 atm. The... 

Gao, J. Combined QM/MM simulation study of the Claisen rearrangement of allyl vinyl ether in aqueous solution, JAm.Chem.Soc., 116(1994), 1563-1564... 

Severance, D. L. and Jorgensen, W. L. (1994) Claisen rearrangement of allyl vinyl ether,in Cramer, C. J. and Truhlar, D. G.(eds.), Structure and Reactivity in Aqueous Solution, American Chemical Society, Washington, pp.243-259. 

A review of Claisen rearrangements in aqueous solution has appeared. The synthesis of natural products utilizing tandem Diels-Alder additions with sigmatropic rearrangement processes has been reviewed, and a brief review of the regioselective synthesis of coumarins, quinolones and thiocoumarins with 3,4-fused pyran or furan ring systems by the Claisen rearrangement has been presented. ... 

Gajewski, J. J. Claisen rearrangements in aqueous solution. Organic Synthesis in Water 1998, 82-101. 

For a discussion on acceleration effects on the Diels-Alder cycloadditions and Claisen rearrangements of hydrophobic compounds in dilute aqueous solutions, see S. Narayan, J. Muldoon, M. G. Finn, V. V. Fokin, H. C. Kolb and K. B. Sharpless, Angew. Chem. Int. Ed., 2005, 44, 3275. 

Numerous organic reactions have been studied in aqueous solutions. It was observed that water is able to induce dramatic rate accelerations in Diels-Alder cycloadditions [66], benzoin condensation [67], Claisen rearrangements [68], Mu-kaiyama aldol reactions [57], Michael reactions [69], Baylis-Hillman reactions [70], and 1,3-dipolar cycloadditions [71], All these reactions are characterized by negative volume changes and negative volumes of activation. It is expected that ground state destabilization in aqueous media associated with transition state stabilization is one of the determining kinetic factors. 

Thus, the 0-methallyl ether of nonylphenol was rearranged by the Claisen reaction to afford an o-hydroxy C-methallyl compound, ethoxylation of which with excess ethylene oxide provided an intermediate for vinylic polymerisation with vinyl acetate in aqueous solution containing ammonium persulphate at 70°C (ref. 25). 

Supportive of the suggestion that ionization is not a major pathway in the Claisen rearrangement of the parent compound is the fact that the SDKIEs in aqueous solution are comparable to those in the gas phase and in m-xylene. Furthermore, attempts to solvolyze 1,1-dideuterioaUyl mesylate in aqueous methanol resulted in no ionization to an allyl cation instead, the direct displacement product was formed exclusively. Finally, determinations of a solvent kinetic isotope effect in deuterium oxide resulted in values around unity 10%. ° In the solvolysis reaction of tert-butyl chloride the value is 40% at room temperature. " It is possible to cause allylvinyl ethers to ionize by providing cation stabilizing substituents and Lewis acids or Lewis acidic solvents. ... 

The course of other types of reactions which are also accelerated in aqueous solutions can help for a better understanding. For instance, in the Mukaiyama reaction between a silylenol ether and a carbonyl compound we obtained, in water, without any catalyst and in a similar yield, the same stereoselectivity than under 1(XX)0 atmospheres in dichloromethane that is the reverse of the normal stereoselectivity obtained by TiC catalysis(22,25). We and others have also described such a rate enhancement in the Claisen rearrangement (24 5), All these reactions, including the Diels Alder reaction have in common their negative activation volume. We anticipate that a reaction under kinetic control, between two small hydrophobic molecules (or between the hydrophobic moieties of amphiphilic molecules) for which the activation volume AVt is negative must be accelerated in water as it is under pressure. When several transition states are possible, the more compact should be favored. Actually, this hypothesis is verified for all our aqueous reactions, especially the Diels-Alder reactions which give virtually pure endo transition state which is also the transition state favored under external pressure. 

Following are given some of the important applications of Claisen rearrangement in aqueous solution. 

LynF, an enzyme from the TruF family, 0-prenylates tyrosines in proteins and subsequent Claisen rearrangements give C-prenylated tyrosine products. The reactions in tyrosines and model phenolic systems have been explored with computational methods. Studies of the orf/to-C-prenylation and Claisen rearrangement of tyrosine and the Claisen rearrangement of a,a-dimethylallyl (prenyl) coumaryl ether establish the energetics of in the gas phase and in aqueous solution (Scheme 29). ... 

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